1. Field of the Invention
The present invention relates to radio receiving apparatus for receiving a variable-length RLC PDU data in an RLC layer belonging to Layer 2 forming a radio communication protocol layer, and more particularly radio receiving apparatus enabling efficiently storage of a received RLC PDU data into a buffer memory.
2. Description of the Related Art
A W-CDMA system becomes widely used today, as a third generation (3G) radio communication system. Further, a standard called HSDPA (High-Speed Downlink Packet Access) comes into practical use to obtain high-speed (14 Mbps maximum) data communication in W-CDMA. HSDPA is also called as 3.5G system because of an improved version of the 3G system. The standardization is carried out by 3GPP (the 3rd Generation Partnership Project), an association for standardizing the 3G system.
HSDPA has the features of (1) shared use of one physical channel by a plurality of mobile terminals (UE) in time division, (2) automatic selection of higher speed modulation system and coding system depending on an electric wave condition, (3) adopting hybrid ARQ incorporating retransmission control (ARQ) combined with correction coding processing, and so on.
FIG. 1 shows a diagram illustrating a data structure of Layer 2 in the protocol architecture corresponding to HSDPA. Layer 2 is divided into sublayers of MAC (Medium Access Control)-hs, MAC-d, and RLC (Radio Link Control).
FIG. 2 shows a diagram illustrating the format of RLC PDU (Protocol Data Unit). RLC PDU shown in FIG. 2 is an Acknowledge Mode RLC PDU enabling data delivery confirmation control and data retransmission control. RLC PDU includes D/C bit for distinguishing between a user data and a control data; a sequence number (SN) indicating the sequential order of RLC-PDU; polling bit P indicating the presence/non-presence of a delivery confirmation request; area HE indicating information of the user data extension area; length indicator LI; E bit; data storage area Data; and padding bit PAD or piggyback (Piggybacked STATUS PDU).
The data size of RLC PDU is fixed to, for example, 42 octets, 82 octets or 122 octets (where 1 octet is 8 bits), which is not changed during communication. RLC PDU is identified by the sequence number SN, which has a numeric value ranging from, for example, 0 to 4,095 maximum.
In RLC shown in FIG. 1, on the transmission side of RLC, a transmission data RLC SDU (Service Data Unit) fed from an upper layer is divided into a plurality of RLC PDUs, and forwarded to the lower MAC-d layer, after a sequence number SN is given to identify each RLC PDU.
Also, on the reception side of RLC, when the RLC PDUs are received from the lower MAC-d layer, by being sorted in order of the sequence number SN, the RLC PDUs are merged to assemble RLC SDU, and then transferred to the upper layer. At this time, when there is a missing sequence number SN, a retransmission request of RLC PDU corresponding to the missing SN is initiated.
Therefore, the transmission side of RLC is required to retain the transmitted RLC PDU in a buffer (memory) until the notification of delivery confirmation is received from the reception side of RLC. Also, the reception side of RLC is required to keep a buffer for the RLC PDU of which SN is missing, when performing RLC SDU assembly.
FIG. 3 shows a diagram illustrating the operation on the reception side of RLC in the HSDPA system specified by 3GPP. In the present and subsequent figures, “H” denotes an RLC Header, and “Data” denotes a Payload.
When a missing sequence number is detected on the reception side of RLC, the transmission side is notified of the missing sequence number SN of the RLC PDU concerned, in the form of a retransmission request. As shown in FIG. 3(a), for example, SN=2 is missing, and the retransmission request therefor is initiated. At this time, on the reception side of RLC, a buffer area for originally storing RLC PDU of SN=2 is kept idle. The buffer size to be kept idle can easily be obtained because of the fixed length of RLC PDU. When the reception side of RLC receives RLC PDU of SN=2 as a retransmission data [refer to FIG. 3(b)], the received PDU is allocated in the buffer area having been kept idle for the PDU concerned, and the assembly of RLC SDU is performed [refer to FIG. 3(c)].
In Japanese laid-open Patent Publication No. 2006-20044, there is disclosed a memory management method in the MAC-hs sublayer, enabling reduction of the increase of the memory amount without need of a complicated memory control method, by dividing a variable-length MAC-hs PDU into each unit of RLC PDU and storing into a shared memory (buffer) together with a sequence number.
After the realization of the above-mentioned 3.5G mobile communication systems by HSDPA, subsequently, migration to the fourth generation (4G) systems will be expected in early stages so as to realize higher speed and larger capacity. However, in the present estimation, one more stage called as “3.9G” (which may also be called as “Super 3G”) will be introduced before migration to 4G systems. As the communication speed of the 3.9G systems, a maximum speed of 100 Mbps, or of that order, is assumed.
In 3GPP at present, as the 3.9G specification, a study is in progress to modify RLC PDU from fixed length, as shown in FIGS. 1, 2, to variable length.
FIG. 4 shows a diagram illustrating an assumed configuration of the RLC sublayer when the RLC PDU is modified to have variable length. As shown in FIG. 4, when the RLC PDU is modified to have variable length, the sequence number SN is used as a number to identify RLC SDU. To identify RLC PDU constituting each RLC SDU, assumedly, an SI (Segment Indicator) is introduced. In case of the fixed-length RLC PDU, the number of RLC PDUs constituting a fixed-length RLC SDU is uniquely determined. Accordingly, the RLC SDU can be identified when the RLC PDU is identified. However, when the RLC PDU is modified to have variable length, the number of RLC PDUs constituting each RLC SDU is not uniquely determined. Therefore, it becomes necessary to introduce any symbol so as to identify RLC SDU further. Thus, the sequence number SN conventionally used to identify each RLC PDU is used as a symbol to identify RLC SDU, and the aforementioned segment indicator SI is newly introduced as the symbol to identify RLC PDU.
FIG. 5 shows an exemplary format when RLC PDU is modified to have variable length. As described above, when RLC PDU is modified to have variable length, the sequence number SN becomes the number to identify RLC SDU, and RLC PDU is to be identified by the combination of the above sequence number SN and the segment indicator SI belonging thereto.
In the format, the reason for providing a plurality of areas for the segment indicator SI is that, when the RLC PDU being divided into a variable length is further divided, additional attachment of the segment indicator SI becomes necessary to identify further divided RLC PDU. At the retransmission control when RLC PDU is modified to have variable length, the following problem will be produced.
FIG. 6 shows a diagram explaining the operation on the reception side of RLC when the RLC PDU is modified to have variable length. When a missing RLC PDU is detected on the reception side of RLC, the transmission side is notified of both the sequence number SN and the segment indicator SI corresponding to the above missing RLC PDU, as a retransmission request. As shown in FIG. 6(a), for example, in case of a missing RLC PDU of SN=0, SI=1, the retransmission thereof is requested. At this time, because the data length of the missing RLC PDU is unknown, it is not possible to keep an idle buffer area for the missing RLC PDU so as to store RLC PDUs in order of the segment indicator SI [refer to FIG. 6(b)].
Assuming to keep the idle buffer area beforehand, it is necessary to assume to receive RLC PDU having a preset maximum length, which results in a high possibility of wasteful buffer area consumption.
In Patent document 1, a variable-length MAC-hs PDU is divided into each fixed-length RLC PDU, which does not produce any problem because the RLC PDU is divided later in the RLC sublayer. However, it is not possible to apply the same method in an upper layer than the RLC sublayer because of no data division performed. Also, in the high-speed communication such as Super 3G, efficient communication can be achieved by elongating each data length, and therefore, it is not preferable to introduce excessive division of data.
FIG. 7 shows a diagram explaining a use state of the buffer area considering the maximum length of the RLC PDU. For example, assuming the maximum length of the RLC PDU is 1,500 bytes, it is necessary to prepare a buffer area of 1,500 bytes for one RLC PDU. Thus, the buffer areas shown by the broken lines in the figure are wasted.
Also, when storing RLC PDUs in a packed manner in order of reception, RLC PDUs having an identical sequence number SN may not always be received consecutively. When RLC PDUs of different sequence numbers SN are received mixed, it is necessary to assemble RLC SDU after extracting-RLC PDUs-having an identical sequence number SN. Thus, the assembly to RLC SDU becomes complicated.
Accordingly, it is an object of the present invention to provide radio receiving apparatus in case of the variable-length RLC PDU, enabling efficient storage of received RLC PDUs into the buffer without wasting the buffer area.